1. Field of the Invention
The subject invention relates generally to antennas and, more specifically, to a method for compensating for failed elements of an active antenna array.
2. Description of Related Art
The advent of active array technology has opened up a new vista of radar detection and tracking possibilities. One now has unprecedented control of phase and amplitude separately on transmit and receive, as well as nearly instantaneous beam positioning.
But with such fine phase and amplitude control comes an additional penalty--namely, an error source due to element failures. In the course of time, such failures accumulate, gradually degrading system performance. At some point, a performance threshold is crossed, necessitating a hardware overhaul.
Basically, element failures produce random amplitude errors in the antenna weighting, causing an average increase in antenna sidelobes. This is especially serious in directions below the horizon due to additional clutter desensitizing medium PRF waveforms. The effect is to reduce detection sensitivity by decreasing the signal-to-junk ratio.
The most general approach to compensating for failed elements uses a steepest descent global compensation in which all elements are adjusted such that the total pattern is minimized in particular directions. This method requires storage of thousands of complex weights computed off-line for each particular configuration of failures.
The disadvantages of global compensation are significant. The steepest descent computation necessary to find the new weights is very time consuming. Hence, it is not generally appropriate for computation in a tactical system and certainly not possible dynamically during beam repositioning. Since previous global compensation strategies involve minimizing energy toward the ground in the entire antenna pattern, there is no easy way of allowing for electronic beam scans for a given set of element failures. The upshot is that massive tables of complex weights must be stored for each configuration of failures, including the thousands of elements and each of the possible scan directions. This solution is not only inelegant in practice, it is infeasible.